use IfElse.jl for Julia 1.6"

[paulflang]

I could imagine that #251 fails on Julia 1.6, because it requires IfElse.jl. Let's see if CI passes for this.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.62%. Comparing base (4eed5fa) to head (aa8856e). Report is 111 commits behind head on master.

Files with missing lines	Patch %	Lines
src/interpret.jl	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
+ Coverage   92.37%   94.62%   +2.24%     
==========================================
  Files          10       10              
  Lines         748      818      +70     
==========================================
+ Hits          691      774      +83     
+ Misses         57       44      -13     

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[exaexa]

closing for inactivity; please feel free to reopen if there's some good replacement for the NaN.

Btw might be useful to also recommend people to override the sbmlPiecewise with watever they like, which could be handled by the docs, is that OK?